The present invention is directed in general to optical devices, and, more particularly, to an optical device including dynamic equalization of optical channel bands for achieving improved performance in a wavelength division multiplexed system.
Optical communication systems are a substantial and fast growing constituent of communication networks. The expression xe2x80x9coptical communication system,xe2x80x9d as used herein, relates to any system that uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber. Such optical systems include but are not limited to telecommunication systems, cable television systems, and local area networks (LANs). Currently, the many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, TDM capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated one of a plurality of channels or wavelengths. The channels are combined by a multiplexer at one end terminal and transmitted on a single fiber to a demultiplexer at another end terminal where they are separated and supplied to respective receivers.
Recently, dense WDM (DWDM) systems transmitting 8 channels on a single fiber have been proposed. These systems can include a demultiplexer having a 1xc3x978 optical splitter, which receives the 8 channels on an input fiber, and outputs the channels on each of 8 outputs. The power level on each of the outputs, however, is approximately xe2x85x9 the input power level. Optical components are respectively coupled to the outputs of the 1xc3x978 splitter for outputting a corresponding one of the 8 channels, which introduce additional loss.
Although 8 channel WVDM systems provide improved capacity, the need for additional capacity has increased with growing Internet traffic and demand for multimedia services. Thus, DWDM systems having higher channel counts are currently being developed. In high channel count systems, however, it is difficult to multiplex and demultiplex a large number of optical channels. For example, in a 40 channel DWDM system, a 1xc3x9740 splitter would be inadequate to demultiplex each of the channels because the power level at each output of such a splitter would be insufficient to maintain an adequate signal to noise ratio. As a result, the transmitted channels cannot be adequately detected. On the other hand, although an optical amplifier could be used to increase the power on the input of the 1xc3x9740 splitter, such an amplifier can be difficult to manufacture, and would fail to provide the requisite optical power per channel at higher channel counts. Moreover, if amplifiers were to be provided at each of the outputs of the 1xc3x9740 splitter, the cost of the demultiplexer would be excessive.
In addition, signal detection in high channel count, high data rate, and long distance systems can be unreliable as a result of varying signal optical power levels resulting from conventional multiplexing and demultiplexing operations. For example, wavelength dependent variations in the gain or transmission characteristics of network elements can result in significant variations in the optical signal power and signal to noise ratio in the transmitted channels. Attempts have been made to introduce gain-flattening filters at multiplexer/demultiplexer outputs to equalize the channel signal powers, but conventional gain flattening filters also display wavelength-dependent variations in output signal power. Thus the difficulties associated with output signal power variations in the transmitted channels persist.
Thus, there is a need for a multiplexer and demultiplexer suitable for incorporation into a high channel count DWDM system which minimizes power loss and enables adequate detection of the transmitted channels. There is also a need for a scaleable DWDM system, which can readily accommodate additional channels with minimal expense. There is also a need for an optical device for providing channel signal power equalization in wavelength division multiplexed optical communication systems.
Consistent with the present invention, an optical device is provided comprising a plurality of separate optical paths, each of which receiving a separate group of optical signals. Each group of optical signals is provided to an associated variable optical attenuator. Separate inputs of an optical combiner are each coupled to an output of an associated one of the variable optical attenuators. The optical combiner has an output providing the separate groups of optical signals in an aggregated form on an aggregate optical signal path.
An optical performance monitor is coupled to the aggregate optical signal path, and is configured to detect an optical signal power of each of the separate groups. The monitor be provided in a variety of configurations, and may include, for example, a spectrum analyzer and an appropriately programmed processor circuit. The monitor supplies a feedback signal to corresponding ones of the variable optical attenuators for adjusting a respective attenuation associated with each of the attenuators in dependence of the detected optical signal powers.
The combiner may be provided in a variety of configurations. In one exemplary embodiment, the combiner may include a plurality of cascaded optical filters, each of the optical filters having an input coupled to an output of an associated one of the variable optical attenuators. Also, an amplifier and/or a gain-flattening filter may be coupled to the aggregate optical signal path.
In a multiplexer configuration, the device may further include a plurality of first optical combiners, each of which being coupled to an associated one of the optical paths for supplying a respective one of the separate groups of optical signals. In a line amplifier configuration, the device may further include an optical separator having a plurality of outputs, each of which being coupled to an associated one of the optical paths for supplying a respective one of the separate groups of optical signals.
In a demultiplexer configuration, a device consistent with the invention may include an optical communication path receiving an optical signal including a plurality of separate wavelengths. The optical performance monitor may be coupled to the optical communication path. An optical separator receives an output of the optical performance monitor, and supplies each of the plurality of groups of the separate wavelengths on a separate associated output. Each of a plurality of variable optical attenuators receives a separate one of the separate associated outputs for adjusting an associated attenuation level.
A method for dynamically equalizing power levels associated with groups of optical signals consistent with the invention includes the steps of: providing each of the groups of optical signals on separate optical signal paths; combining each of the groups of optical signals on an aggregate optical signal path; detecting the power level associated with each of the groups of optical signals on the aggregate optical signal path; and attenuating each group of the optical signals on the separate signal paths in dependence of an associated detected power level. In a demultiplexer application, another exemplary method of dynamically equalizing power levels associated with groups of optical signals, includes the steps of: providing the groups of optical signals in an aggregated form on an optical signal path; detecting the power level associated with each of the groups of optical signals on the optical signal path; separating the groups of optical signals onto associated separate optical signal paths; attenuating each group of the optical signals on the associated separate signal paths in dependence of an associated detected power level.